The present invention relates to a lithium secondary battery that can be easily manufactured, which has small internal resistance, and is provided with a safety mechanism for breaking a current at an abnormal time such as temperature rise in the battery due to over-charging/discharging.
A lithium secondary battery has attracted a great deal of attention as a secondary battery that is small, which has a large energy density, and can be charged and discharged, and which can be used as a power source for an electric equipment such as a portable communication equipment and a notebook-sized personal computer, the electric equipment being rapidly downsized in recent years. Moreover, concerns for resource saving and energy saving are raised on the background of internationally protecting the earth environment, and this is one of the reasons why the lithium secondary battery is expected as a motor driving battery for an electric vehicle. Further, considerable concerns are given to putting a large capacity lithium secondary battery into early practical use for the purpose of using it as an emergency backup power source for a large communication equipment or a host computer which supports an information society.
Especially, as to automobiles, the movement of switching into alternative energy that can reduce a large amount of consumption of fossil fuels such as gasoline has rapidly spread for these several years. For example, introduction of electric vehicles is regulated by law in many countries and many regions. As a battery for such an electric vehicle, conventionally, a lead acid storage battery has been mainly used. However, since the lead acid storage battery itself is heavy and requires a large capacity, it has been difficult to realize driving performance comparable to a gasoline fueled vehicle. However, it has been achieved to provide lithium secondary battery having a high performance in recent years, and the realization of electric vehicles has taken shape.
Although the structure of such a large lithium secondary battery has not been reported openly, it is possible to basically make its structure similar to a small lithium secondary battery. That is, in a small lithium secondary battery, as shown in FIG. 3, in an electricity generating portion, positive electrodes 50 and negative electrodes 51 including negative lead portions 52 are wound through separators 53, and the wound member is inserted into a metal battery case 1 so as not to cause the positive electrodes 50 to be electrically connected through the battery case 1 by an insulating film 54. The negative lead portions 52 are connected to the inside of the metal battery case 1. A positive lead 3 is attached to the positive electrodes 50, and an insulating plate 55 is provided so that the positive lead 3 is not brought into contact with the negative electrodes 51. A non-aqueous electrolytic solution in which an electrolyte is dissolved, is filled in an electrode wound portion.
The lithium secondary battery is a secondary battery that can be charged and discharged, which has properties that the potential is higher than conventional secondary batteries, and the energy density is high. Thus, the lithium secondary battery is provided with a safety mechanism for preventing accidents such as a burst of the battery caused from rising of the battery temperature by abnormalities at the time of charging and discharging, for example, over-discharging due to a short circuit of external terminals, abnormalities in rapid charging or over-charging due to malfunction of a charging device, application of a reverse connection potential by erroneous use of a user, and the like. That is, as shown in FIG. 2, one end of the positive lead 3 connected to the positive electrodes 50 is connected to an internal terminal 4 having a pressure releasing hole 5, and the other end of the positive lead 3 is connected to the positive electrode. The internal terminal 4 is electrically connected to a pressure switch plate 7 having a burst groove 9 through a contact B, and further, the pressure switch plate 7 is connected to an external terminal 15 through a PTC positive temperature coefficient element 21. The internal terminal 4 and the pressure switch plate 7 are isolated from each other by an insulator 6 so that when the contact B separates by the increase of an inner pressure of the battery, the electrical connection is lost. All these parts are contained in the battery case 1.
In case that the temperature of the battery itself having such a safety mechanism is raised by the occurrence of abnormal state upon use of the battery, when the PTC element 21 reaches a predetermined temperature, the resistance thereof abruptly increases so that a current hardly flows. Thus, a battery reaction is suppressed and the rising of the battery temperature is suppressed. In this case, as this kind of PTC element, a mixture of conductive particles and polymer is generally used. In other words, such a mixture has low resistance at room temperature since the conductive particles form a conductive path, while at a temperature higher than a predetermined temperature, the mixture comes to have high resistance close to an insulator since the molecular structure of the polymer is changed so that the conductive path formed of the conductive particles is cut to pieces, and when the temperature decreases again, the polymer returns to the original structure so that the conductive path of the conductive particles is formed and the resistance thereof is returned to a low value.
However, if the internal pressure of the battery is raised notwithstanding that a current is restricted by the PTC element 21, and the internal pressure exceeds the welding strength of the contact B between the pressure switch plate 7 and the internal terminal 4, the contact B separates so that the connection between the electricity generating portion and the external terminal is completely interrupted and the battery reaction does not occur. Further, nevertheless, if the internal pressure is raised, the burst groove 9 bursts so that the internal pressure is released to the atmospheric pressure. Such a mechanism is conventionally adopted.
In addition to the above described current control mechanism, a safety mechanism is provided such that by using a microporous film, such as polyethylene having a low softening point, as the separator for separating the positive electrode from the negative electrode in the electricity generating portion; when the battery temperature is raised, the separator film is softened and micropores formed in the film are collapsed, so that the movement of lithium ions is blocked and the battery reaction is suppressed.
However, for a large battery the resistivity of the above-mentioned PTC element made of the conductive particles and polymer is too large, about 1.OMEGA..multidot. cm at room temperature. So the PTC element may cause the large output loss and the short lifetime. Especially, in the case where such a PTC element is mounted on a large battery, concentration of current inside the PTC element is apt to occur due to the increase of an area of the element, which causes heat generation, so that the mounting to a large battery is difficult. In addition, the PTC is generally expensive, and a large-sized one is not manufactured. Accordingly, a current control element which is more inexpensive, is capable of dealing with a large battery, and has low resistance, is earnestly desired.
Further, it is thought that if a large battery is erroneously treated, the scale of accidents would become larger and the danger derived therefrom would also become higher than that imagined from a small battery. So it is considered to be desirable to make the safety standard for a large battery stricter than that of a small battery. At present, in a PTC used for a small lithium secondary battery, the transition temperature at which the resistance of the PTC abruptly increases is about 130.degree. C. For a large lithium secondary battery, this transition temperature about 130.degree. C. is too high. So a material or a mechanism to break current at a temperature lower than this temperature is required.
In addition to such problems of the prior art, it is considered to be necessary to contrive means such that the resistance of an electricity generating portion itself is made small to widen the selection of materials of positive electrodes and negative electrodes. Also, efficient electricity collection from the electricity generating portion to the outside terminal is effective in improving the performance of the battery. Moreover, in order to put a battery into wide use, it is also necessary to lower the manufacturing cost, and it is preferable that a battery having a simple structure is manufactured by a simple method.